1. Field of the Invention
The present invention relates to conductive elastomeric foams. In particular, the present invention relates to methods of rendering insulative polymeric foams conductive.
2. Brief Summary of the Related Art
Conductive elastomers are well known for use in soling materials for shoes, in electrostatic dissipative (ESD) footwear, and in other applications which require ESD and Electromagnetic Interference/Radio Frequency Interference (EMI/RFI) shielding. Cellular elastomers, such as polyurethane foams, are presently rendered conductive by the incorporation of ionic compounds, but this approach has a number of drawbacks. The most serious of these is that the lowest electrical resistance (highest conductivity) achievable is a conductivity of about 1.times.10.sup.-8 Siemens per centimeter (S/cm), the resistivity is also very sensitive to temperature and humidity (hygrothermal conditions), and the conductivity changes over time due to ion mobility and ion depletion.
Use of electrically conductive fillers can overcome some of these disadvantages, but it is difficult to achieve a target resistivity with conductive fillers in, for example, the 1.times.10.sup.-6 to 1.times.10.sup.-7 S/cm conductivity range due to the percolation threshold mechanism. At the percolation threshold concentration of an electrically conductive filler, the conductivity increases rapidly from that of an insulator to that of a semiconductor. As a result, the conductivity is very sensitive to small variations in filler concentration at the percolation threshold concentration. Moreover, resilient foams cannot be obtained in the range of conductivity required for Electromagnetic Interference/Radio Frequency Interference shielding applications, that is, most typically greater than about 0.10 S/cm.
Inherently conductive polymers may also be used in the form of blends or composites with other polymers, providing conductivity as well as improved mechanical properties and processability characteristics to the conductive blends or composites. During the past fifteen years, there has been significant progress in the synthesis of conductive organic polymers having novel electrical, optical and electrochemical properties. Among those conducting polymers, polypyrrole has been especially promising for commercial application, because of its good environmental stability and ready synthesis. Polypyrrole is ordinarily synthesized by either oxidative, chemical or electrochemical polymerization of pyrrole.
While polypyrrole is conductive, it is also insoluble and non-fusible. Polymerization of .beta.- or N-substituted pyrrole with alkyl chains having more than six carbons yields polymers with improved solubility in organic solvents. However, because of stearic and/or electronic interactions, the conductivity of these substituted polypyrroles depends on the position of the alkyl substitution. The P-alkyl substituted polypyrroles thus exhibit conductivities one to two orders of magnitude lower than polypyrroles, and N-alkyl substituted polypyrroles have conductivities about five to six orders of magnitude lower than polypyrrole.
In addition to polypyrroles, copolymers of pyrrole and N-substituted pyrroles have advantageous conductive and other properties. The monomer oxidation potentials of pyrrole (1.15 volts (v) vs. Standard Colomel Electrode (SCE)) are very close, which indicates that the monomers have very similar polymerization reactivity. However, the polymer redox potential for polypyrrole about is 0.5 V less than poly(N-methylpyrrole) ("PMPy"), which indicates that polypyrrole is more oxidatively stable than poly(N-methylpyrrole). The conductivity of the copolymer depends upon the composition and is intermediate between that of polypyrrole (10-100 S/cm) and poly(N-methylpyrrole)(10.sup.-4 -10.sup.-5 S/cm).
Blends or composites of conductive polymers with other polymers are manufactured by either dispersing conducting polymer particles directly into an insulating polymer matrix, or by an in situ polymerization of the conducting polymer within a polymer host. The in situ polymerization of pyrrole by vapor phase polymerization within a polymer matrix containing a suitable oxidant has been reported. This approach has been used to prepare conductive blends based on a number of different polymer matrices, including poly(vinylchloride), poly(vinyl alcohol), cotton, poly(phenylene terephthalamide), and polyurethane as are described in "Conductive Polymer Blends Prepared by In Situ Polymerization of Pyrrole: A Review", Polymer Engineering and Science, December 1997, Vol. 37, No. 12, 1936-1943, the disclosure of which is incorporated herein by reference.
All of these polymers are dense polymers, that is, none are in the form of a foam. Foam presents particular problems for in situ vapor phase polymerization, because the conducting polymer must be restricted to the walls or struts of the foam. If the conducting polymer forms on the surface of the foam cells, it is too easily removed by abrasion or handling the foam. Formation of the polymer on the surface of the foam cells not only decreases the conductivity of the composites, but also may result in undesirable marking of surfaces which come into contact with the foam. (See "Preparation of Polypyrrole-Polyurethane Composite Foam by Vapor Phase Oxidative Polymerization", by He et al., Journal of Applied Polymer Science, Vol. 55, 283-287 (1995)). Accordingly, there remains a need in the art for a method of effective in situ polymerization of pyrroles and other monomers within a foam which result in stable, conductive blends and composites.